1. FIELD OF THE INVENTION
The present invention relates to an outboard engine structure and, more particularly, to an outboard engine structure in which a crankcase of a vertical engine is connected to the upper surface of an extension case of which a hollow portion leads to an exhaust port located below a draft line and a crankshaft of the engine is coupled with a driving shaft for driving a propeller shaft arranged in the hollow portion and an exhaust system of the engine opens into the hollow portion.
2. DESCRIPTION OF THE PRIOR ART
In a conventional outboard engine structure of this type, as disclosed, for example, in Japanese Patent Laid-open No. 43895/1983 official gazette, the position of a cylinder head of the engine is substantially displaced from the center line of a hollow portion of an extension case. Thus, in order to lower the temperature of the exhaust gas of the engine and guide the gas to the hollow portion, an exhaust port is extended to the lower surface of the crankcase coupled to the upper surface of the extension case, an exhaust manifold leading to the exhaust port is suspended in the hollow portion, and both ends of the manifold are coupled to the crankcase and the extension case.
When an exhaust system of such conventional outboard engine structure is subjected to maintenance and inspections, such drawbacks and disadvantages arise that the entire engine must be removed from the extension case, resulting in lowering of the maintainability of the engine, and that the exhaust system has a complicated structure and requires a high manufacturing cost.
In the conventional engine of this type, a water jacket is formed to cover a wide range over a combustion chamber formed in the cylinder head as well as a lower portion of a cylinder bore formed in the cylinder block and having a piston slidably fitted therein and an inlet passage of the water jacket is formed in the crankcase connected with one side of the cylinder block.
However, in the engine of this type, cold water such as sea water is always supplied as coolant to the water jacket. Thus, when the engine employs the conventional water jacket and the inlet passage, cold water first flows into the inlet passage to cool the crankcase which is relatively unheated. Accordingly, the crankcase is overcooled to delay the temperature rise of lubricating oil in the crankcase, thereby extending a warming up operation of the engine.